What is unwrap() in Rust and what is it used for? Explain with an example

Hello! I've just started learning Rust and I constantly see the `.unwrap()` method being called in other people's code. For example, when reading a file or parsing a number from a string. I understand it has something to do with errors, but I don't fully understand it. Here's an example I found in the documentation: ```rust use std::fs::File; use std::io::Read; fn main() { let mut file = File::open("hello.txt").unwrap(); let mut contents = String::new(); file.read_to_string(&mut contents).unwrap(); println!("{}", contents); } ``` What does `unwrap()` do here? Why can't you just get the value without it? And what happens if the file doesn't exist? I've heard that beginners are advised not to use `unwrap()` in production code, but then how should such situations be handled properly? Please explain in simple terms.